Rutting on asphaltic pavement has been a significant concern of highway commissions in the United States. Rutting is caused by a load passing repeatedly across the surface of pavement and results in lateral spreading of the pavement from the location of load application, thereby producing a rut or groove. This problem has increased in severity as the wheel loads and truck traffic on highways have increased. Rutting may cause roads and highways to become non-serviceable and dangerous. If excessive, rutting may produce hazardous conditions, such as increasing the likelihood of vehicle hydroplaning due to water accumulating in the formed rut.
Because of these problems and dangers, highway commissions in the United States and throughout strive to install the safest and most economical pavement available. Hence, test specimens of the prospective pavement material must be tested during the developmental stages, i.e., prior to construction, to determine the susceptibility of a given material to damage by rutting.
Various devices and test methods have been proposed for testing asphalt pavement. For instance, U.S. Pat. No. 3,119,257 to Speer is directed to a traffic simulator apparatus and method. The test track of that apparatus is polygonal and is formed of plural elongated, substantially rectangular paving strips joined end to end. A central column including a turntable apparatus from which radially arms extend therefrom are provided wherein each arm retains a wheel, preferably an actual rubber tire. The central column is rotated so the wheels traverse the upper surface of the polygonal track. The apparatus of Speer is extremely large, making it impractical in a laboratory environment.
The Georgia Loaded Wheel Testing Apparatus, developed by James S. Lai, Professor School of Civil and Environmental Engineering, Georgia Institute of Technology with the cooperation of the Georgia Department of Transportation, was initially developed in 1985 and modified in 1992. The Georgia Loaded Wheel Testing Apparatus is a two part system. A compactor creates a compacted hot mix asphalt specimen and a wheel tracking system repeatedly applies a load to the specimen. The wheel tracking system includes a slidable tray for retaining a beam sample tray for holding the samples. The slidable tray is associated with a reciprocating drive assembly. An inflated tube is positioned over the specimen and a weighted wheel is positioned thereover wherein the specimens are rotated to create rutting. The wheels are thus passed in a reciprocating or side to side manner, whereby the time and velocity of the load application across the specimen is sinusoidal. This type of load, however, is dissimilar to actual loads applied to pavement. Furthermore, the reciprocating motion of the specimen is an inefficient use of energy when the motion is driven by a rotating motor.
Other attempts related to asphaltic pavement testing include the Hamburg Wheel Tracking Device manufactured by Helmut-Wind, Inc. of Hamburg, Germany. The Hamburg Device is used to measure moisture sensitivity as well as rutting susceptibility. A weighted steel wheel is rolled back and forth over a compacted specimen which is immersed in water. The rate of deformation is used to determine the susceptibility to water damage, stripping, and rutting. Stripping results from the direct contact of the steel wheel against the surface of the specimen. Like the Georgia Loaded Wheel Tester, the wheels of The Hamburg Device are reciprocated across the surface of the test specimen resulting in a load wherein the time and velocity of the load application upon the specimens is sinusoidal.
These and other prior art approaches have many shortcomings or limitations which significantly restrict their usefulness or practicality as a laboratory test instrument for achieving accurate and reproducible evaluations of the susceptibility of a test specimen to rutting.